1. Field of the Invention
The invention relates particularly to the field of dielectric fluids used for thermal management of transformers. More particularly, it relates to improved compositions that provide both electrical insulation and/or heat dissipation for transformers and other apparatus.
2. Background of the Invention
Thermal management of transformers is known to be critical for the safety of transformer operation. Although conventional transformers operate efficiently at relatively high temperatures, excessive heat is detrimental to transformer life. This is because transformers contain electrical insulation which is utilized to prevent energized components or conductors from contacting, or arcing over, the other components, conductors, or internal circuitry. In general, the higher the temperatures experienced by the insulation, the shorter its life. When insulation fails, an internal fault or short circuit, sometimes leading to fire, may occur.
In order to prevent excessive temperature rise and premature transformer failure, transformers are generally filled with a liquid coolant to dissipate the relatively large quantities of heat generated during normal transformer operation. The coolant also functions to electrically insulate the transformer components as a dielectric medium. The dielectric liquid must be able to cool and insulate for the service life of the transfer, which is in a number of applications in excess of twenty years. Because dielectric fluids cool the transformer by convection, the viscosity of a dielectric fluid at various temperatures is one of the key factors in determining its efficiency.
Mineral oils have been tried in various dielectric formulations, particularly because they may offer a degree of thermal and oxidative stability. Unfortunately, however, mineral oils are believed to be environmentally unfriendly and may exhibit unacceptably low fire points, in some cases as low as 150 degrees Celsius (° C.) which is undesirably close to the maximum temperatures to which a dielectric fluid is likely to be exposed during use in a given application, such as a transformer. Because of their low fire points, researchers have sought alternative dielectric materials.
In this search for alternatives, vegetable oils were early-identified as a dielectric medium that could be environmentally friendly and exhibit the desired characteristics of desirably high fire points (significantly greater than 150° C.) and desirable dielectric properties. They may also be biodegradable within a short time. Finally, they may offer enhanced compatibility with solid insulating materials.
Unfortunately, vegetable oil based fluids may suffer from their own drawbacks when compared with mineral oils. For example, vegetable oils may tend to have higher pour points, e.g., greater than 0° C. This is problematic for the many applications where a pour point at or below −15° C. may be required. They may also have an undesirably higher viscosity than a mineral oil based fluid. Thus, researchers seek to identify dielectric fluids that can operate safely and properly within a broad temperature range of from about −15° C. to about 110° C., and which are thermally and oxidatively stable therein.
Researchers looking for alternative have identified a number of possible fluids. For example, U.S. Pat. No. 6,340,658 B1 (Cannon et al.) describes a vegetable oil-based electrically-insulating fluid, which is environmentally friendly and has a high flash point and high fire point. The base oil is hydrogenated to produce maximum possible stability of the oil. Vegetable oils are selected from, e.g., soybean oil and corn oil.
U.S. Patent Publication 2008/0283803 A1 describes a dielectric composition comprising at least one refined, bleached, winterized, deodorized vegetable oil and at least one antioxidant. The dielectric fluid further comprises at least one synthetic ester, wherein the synthetic ester is a bio-based material. The patent defines the term “synthetic ester” as referring to esters produced by a reaction between (1) a bio-based or petroleum derived polyol: and (2) a linear or branched organic acid that may be bio-based or petroleum derived. The term “polyol” refers to alcohols with two or more hydroxyl groups. Suitable examples of the bio-based synthetic esters included are those produced by reacting a polyol with an organic acid with carbon chain lengths of C8-C10 derived from a vegetable oil such as, for example, coconut oil. The synthetic esters also included synthetic pentaerythritol esters with C7-C9 groups. Other polyols suitable for reacting with organic acid to make the synthetic esters include neopentyl glycol, dipentaerythritol, and e-ethylhexyl, n-octyl, isooctyl, isononyl, isodecyl and tridecyl alcohols.
Despite these and other efforts by a variety of researchers, there is still a need to develop dielectric fluids that have the desired combination of properties as well as economic viability and capability for biodegradation.